1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus for storage, retrieval and deployment of vertically suspended drag gages used in a semi-automated system for inspecting a fuel assembly for potential control rod guide thimble misalignment.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels or guides for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
In addition to accommodating normal stepped insertion of the control rods, the guide thimbles must allow their rapid insertion should a reactor trip arise. Therefore, one procedure typically carried out to determine the quality of a fuel assembly is an inspection for potential control rod hang up or malfunction by performance of a drag check utilizing tooling rods. The slender tooling rods, referred to collectively as a drag gage, are lowered into the hollow control rod guide thimbles in a vertical attitude. The descending gage must not encounter a drag which results in a weight reduction of fifteen pounds or more to qualify the fuel assembly as acceptable.
Heretofore, this inspection task has required use of the main bridge crane and three persons to carry it out. Particularly, in addition to a person to operate the inspection equipment, the inspection procedure has utilized the main bridge crane with the assistance of a crane operator and a hook-up man stationed on an elevating lift platform. Thus, the current inspection task is labor intensive and occupies the crane for prolonged periods of time. Consequently, a need has emerged to improve and automate the manner in which the inspection is carried out.